Microwave attenuator or modulator



y 1961 A. LEDERMAN 2,985,853

MICROWAVE ATTENUATOR OR MODULATOR Filed Jan. 13, 1958 Sol/RC5 APPA RA r116 INVENTOR ALBERT LEDERMAA/ ATTO NEY United States Patent MICROWAVE ATTENUATOR OR MODULATOR Albert Lederman, New York, N.Y., assign'or to Microwave Semiconductor & Instruments Inc., Richmond Hills, N.Y., a corporation of Delaware Filed Jan. 13, 1952;, Ser. No. 708,641

3 Claims. Cl. 333-81 The present invention relates to microwave devices and in particular to transmission components for microwave energy.

It has been known for some time that a body of ferrite in a microwave field will significantly affect the microwave transmission when an external magnetic field is impressed on the ferrite. A discussion of ferrite properties will be found, for example, in the Bell System Technical Journal, vol. 34, pages 5-403, January 1955. By varying the impressed magnetic field, the microwave transmission can be varied. The phenomenon has been variously applied, as in wave-guides and in antennas. In usual coaxial and wave-guide devices, the ferrite is contained in the transmission path, in the space between the conductive walls that outline the path. In antennas, the ferrite is usually coupled to the wave-guide as a dielectric radiating stub and thus forms the transmission path.

An object of the present invention is to provide microwave ferrite devices and apparatus of novel construction. A further object is to provide novel microwave apparatus and transmission devices having operating characteristics that are useful and different from those here tofore obtained with devices having ferrite bodies within the microwave transmission space.

I have discovered that it is possible to obtain a marked interaction between a ferrite body and microwave energy in a space that is bounded by a metallic film carried by the ferrite body, where the ferrite is outside the transmission space. This concept is of considerable interest and advantage inasmuch as the introduction of a ferrite body into a wave-guide, for example, introduces a prominent impedance change (inasmuch as the ferrite has a high dielectric constant and a permeability that differ from the remainder of the system in which it is used), and gives rise to relatively diflicult matching problems.

The design problems and the operating characteristics of such known devices will be found to be quite different than in the case where the ferrite is used for its customary effect but is disposed outside the transmission space. The metal film, arranged to constitute a boundary of the transmission path, is made so thin that interaction of the microwave energy and the ferrite still takes place. This thickness may be variously chosen in relation to the microwave transmission frequency, as being less than the depth of penetration of the microwave current, that is,

less than /i time where w is 21r times the microwave frequency, ,1. is permeability of the film and a is its conductivity; and it may also be operationally related to the achievement of the desired interaction between the microwave energy in the transmission path and the ferrite outside that path. For an operating frequency of kmc., the depth of penetration in copper is 7 10 cm. For the foregoing interaction to occur at this frequency, a film of copper should be substantially thinner than this, as 10- cm. for example.

The disclosure of an illustrative embodiment of the invention which follows below is drawn to a coaxial transmission line in which the inner conductor is in the form of a film of metal, carried by a ferrite rod. A coil surrounds the unit in this embodiment for impressing an axial magnetic field on the rod.

Such a device has varied uses, typically as a modulator and as an electronically controllable attenuator. When the current through the coil is varied, the ferrite, interacting with the microwave energy in the space between the inner and outer coaxial conductors, varies the phase and amplitude of the microwave transmission. As applied to antennas, the radiation pattern can be varied under electrical control.

In the coaxial device that is detailed below, the main transmission path is bounded by an outer metal cylinder and by a film of metal that is supported mechanically by a ferrite rod, the film being the inner conductor of the coax. Microwave currents carried by the thin film are intimately coupled magnetically to the ferrite, which in turn influences the microwave currents carried by the film and the electromagnetic transmission. A like effect can be obtained with other forms of microwave transmission devices, such as a rectangular wave-guide having one or more bounding walls defined by a thin film as above, carried on the surface of a ferrite body outside the transmission space. In the illustrative device below, the E-vector is perpendicular to the film; and in such other microwave transmission devices the excited mode may be arranged to produce at least a component of the E-vector perpendicular to the film-on-ferrite walls.

In another aspect, the invention resides in the provision of a novel structure in which a controlled intimate interaction is produced betwen microwave currents in a metal film less than skin depth in thickness and a ferrite body bearing such film. Such film may be a continuous covering in which the ferrite is located outside the transmission path, the ferrite provides physical support for the film, and the coupling between the ferrite and the current carried by the film is intimate.

The accompanying drawing is a single figure which is a longitudinal cross-section of a novel ferrite device embodying features of the present invention, diagrammatic in matters of detail, and this figure includes diagrammatically illustrated input, output and control apparatus.

In the drawing, a signal source 10 of microwave energy is shown connected to the inner conductor 12 of a conventional coaxial line having an outer conductor 14 in the form of a metal cylinder. These two are mechanically spaced by a suitable insulating support 16. An output device 18, whose nature depends on the application, is shown connected to inner conductor 12 and outer conductor 14' that are coupled to the corresponding conductors 12 and 14 of .the coaxial line excited by source 10. These elements 12' and 14 are mechanically separated by an insulator 16'. Outer cylindrical conductors 14 and 14' are directly connected to each other and they are mechanically joined by a threaded collar 20.

The ends of inner conductors 12 and 12 are spaced apart, and are interconnected by a component 22 including a body 24 of ferrite having cupped cylindrical end fittings 26 of metal which plug into complementary receptacles in the ends of conductors 12 and 12, and a film 28 of metal or other conductor bridges metal fittings 2'6 and covers body 24 of ferrite. The thickness of metal film 28 is less than the depth of penetration at the microwave frequency, as discussed above. Expressed otherwise, the thickness is such that interreaction occurs between the microwave energy that passes through the transmission space between the outer conductor 14 and the metal film 28, and the ferrite 24 which is outside of that space.

Coil 30 surrounds the outer conductor 13 and is coaxial with it, and a source of magnetizing current 32 is provided for this coil 30. This may be of a form to provide an adjustable direct current, or it may deliver a modulated current.

The coaxial elements 12 and 14 have the same radii as the coaxial elements 22 and 14, so that the geometries .of the two portions of the coaxial transmission system are the same and the characteristic impedance of the two portions of the transmission system are basically the same; and the transmission space between the inner and outer conductors are free of inserted ferrite.

The device described is inherently an attenuator that may be compared to a like device in which the ferrite is replaced by an insulator such as glass; but the attenuation of the present device varies more significantly over a frequency range for a given film thickness, without reliance on the magnetizing coil.

When a magnetic field is impressed on rod 24, variation of the impressed field can be used to control or to modulate the transmission. An increased impressed field will result in greater attenuation, up to a limiting value of field, after which further increase in impressed field produces progressively reduced attenuation. A similar effect may be produced in a wave-guide having a ferrite wall outlining a transmission path, in which the inner surface of the ferrite wall bears a film as described above.

I do not know of a positive explanation of the phenomenon that is involved in the operation of the device described above. It may be that there is penetration of the main transmission through the layer 28. In this coaxial transmission line, the E-vector is radial, and there may be a penetration of this vector through this metal film 28 and into the ferrite. Accompanying the penetration by the E-vector there should also be a magnetic component which would explain the effect. Another explanation may be that the thin film 28 is so thin as to be an imperfect shield and the current carried by this film 28 may propagate microwave energy in the ferrite, to produce the observed efiect. These are the best explanations that I can now provide, but I do not intend that these shall be limiting. Whatever the true explanation may be, it is an observed fact that interaction does occur between the microwave transmission along the coaxial system and the magnetic field imposed by coil 30 on the ferrite body 24.

Further applications of the invention will readily occur to those skilled in the art, and the illustrative embodiment described is susceptible of a wide latitude of modification. Consequently, this invention should be broadly construed in accordance with its full spirit and scope.

What is claimed is:

1. Microwave apparatus having means including a pair of space-bounding elements for defining a transmission path therebetween and microwave exciting means for propagating microwave energy along said path, a ferrite support outside said path and a metallic transmission-space-bounding film in the form of a substantially continuous covering on said support, said metallic film constituting one of said space-bounding elements, the thickness of the film being less than skin depth at the microwave frequency, and an electromagnet assembled to said elements for impressing controllable magnetization on said ferrite support.

2. A microwave device including a cylindrical outer conductor, a component coaxially supported within the outer conductor, said component including a rod of ferrite and a substantially continuous covering layer of metal on and in intimate contact with the rod extending from end to end thereof, and a coil coaxially about the outer conductor, the thickness of the metal layer being less than skin depth at the microwave transmission frequency.

3. A microwave device including a pair of coaxial metal elements defining a microwave transmission space therebetween, a coil about the outer element, one of said elements being a body of ferrite outside of said transmission space and bearing a metal film in the form of a substantially continuous covering whose thickness is less than skin depth at the microwave transmission frequency, said film being in intimate contact with said body of ferrite and defining one of the boundaries of the coaxial space.

References Cited in the file of this patent UNITED STATES PATENTS 1,781,124 Nein Nov. 11, 1930 2,197,123 King Apr. 16, 1940 2,529,436 Weber et a1. Nov. 7, 1950 2,669,603 Prache Feb. 16, 1954 2,755,447 Engelmann July 17, 1956 2,784,378 Yager Mar. 5, 1957 2,887,665 Suhl May 19, 1959 2,943,274 Duncan June 28, 1960 FOREIGN PATENTS 261,233 Switzerland Aug. 16, 1949 1,089,421 France Sept. 29, 1954 1,002,417 Germany Feb. 14, 1957 1,141,972 France Mar. 25, 1957 OTHER REFERENCES Whinnery et al.: Coaxial-Line Discontinuities, Proceedings of the I.R.E., vol. 32, No. 11, November 1944, pages 695709.

Burgess: Proceedings of the I.R.E., vol. 44, No. 10, October 1956, pages 1460-4462. 

